LTC1291 datasheet(15/20 Pages) LINER

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LTC1291

APPLICATI

I FOR ATIO

or GND lead will cause gain errors and offset errors (Figure

7). For the best performance the LTC1291 should be

soldered directly to the PC board. If the source can not be

placed next to the pin and the gain parameter is important

the pin should be Kelvin-sensed to eliminate parasitic

resistances due to long PC traces. For example, 0.1

Ω of

resistance in the VCC lead can typically cause 0.5LSB

(ICC × 0.1Ω/VCC) of gain error for VCC = 5V.

When the input MUX is selected for single-ended input the

minus terminal is connected to GND internally on the die.

Any parasitic resistance from the GND pin to the ground

plane will lead to an offset voltage (ICC × RP2).

“+” Input Settling

The input capacitor is switched onto the “+” input during

the sample phase (tSMPL, see Figure 9). The sample period

is 2.5 CLK cycles before a conversion starts. The voltage

on the “+” input must settle completely within the sample

period. Minimizing RSOURCE+ and C1 will improve the

settling time. If large “+” input source resistance must be

used, the sample time can be increased by using a slower

CLK frequency. With the minimum possible sample time

of 2.5

µs, RSOURCE+ < 1.0k and C1 < 20pF will provide

adequate settle time.

“–” Input Settling

At the end of the sample phase the input capacitor switches

to the “–” input and the conversion starts (see Figure 9).

During the conversion, the “+” input voltage is effectively

“held” by the sample-and-hold and will not affect the

conversion result. It is critical that the “–” input voltage be

free of noise and settle completely during the first CLK

cycle of the conversion. Minimizing RSOURCE– and C2 will

improve settling time. If large “–” input source resistance

must be used, the time can be extended by using a slower

CLK frequency. At the maximum CLK frequency of 1MHz,

RSOURCE– < 250Ω and C2 < 20pF will provide adequate

settling.

Input Op Amps

When driving the analog inputs with an op amp it is

important that the op amp settles within the allowed time

(see Figure 9). Again the “+” and “–” input sampling times

can be extended as described above to accommodate

slower op amps. Most op amps including the LT1006 and

LT1013 single supply op amps can be made to settle well

even with the minimum settling windows of 2.5

µs (“+”

input) and 1

µs (“–” input) that occurs at the maximum

clock rate of 1MHz. Figures 10 and 11 show examples

adequate and poor op amp settling.

and capacitances will slow the settling of the inputs. It is

important that the overall RC time constant is short

enough to allow the analog inputs to settle completely

within the allowed time.

Figure 7. Parasitic Resistance in the VCC and GND Leads

Source Resistance

The analog inputs of the LTC1291 look like a 100pF

capacitor (CIN) in series with a 500Ω resistor (RON). CIN

gets switched between “+” and “–” inputs once during

each conversion cycle. Large external source resistors

Figure 8. Analog Input Equivalent Circuit

REF+

REF–

D/A

VCC

GND

LTC1291 F07

RP1

RP2

LTC1291

3RD CLK

↑

RON = 500Ω

CIN =

100pF

LTC1291

“+”

INPUT

RSOURCE +

VIN +

“–”

INPUT

RSOURCE –

VIN –

LTC1291 F08

5TH CLK

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